Conventionally, a cruise control system (referred to as “CC system” hereinafter) has been developed as a system that performs a constant vehicle speed travel for maintaining a vehicle speed at a set vehicle speed. According to this system, the driver may depress a predetermined switch when the vehicle speed reaches a desired speed by means of the driver's acceleration/deceleration operation upon the accelerator pedal, so that the vehicle speed may be set to the speed at that moment when the switch has been depressed and thereafter the vehicle may travel constantly at the set vehicle speed. During this “constant speed control mode”, since the vehicle speed can be maintained without the driver's depression on the accelerator pedal, the driver's burden could be reduced especially when traveling through expressways.
On the other hand, recently there has been proposed and developed a cruise control system equipped with a vehicle-to-vehicle distance control capability (this system is called “adaptive cruise control system”, which will be simply referred to as ACC system hereinafter). In the ACC system, a preceding vehicle is detected by means of radar, camera and the like, and if any preceding vehicle is not detected, the system performs a “constant speed travel” in which the set vehicle speed is maintained, but if the preceding vehicle is detected, it performs a “constant vehicle-to-vehicle distance travel” in which the set vehicle-to-vehicle distance is maintained. Such travel mode in which the vehicle-to-vehicle distance and the vehicle speed are adjusted depending on whether the preceding vehicle exists or not will be referred to as “vehicle-to-vehicle distance control mode” hereinafter.
Japanese Patent Application Laid-open No. H11-42957 discloses an example of such cruise control system that implements a vehicle-to-vehicle distance control mode. This system is so designed that the driver can select and set the desired distance by switching to either of three predefined stages of vehicle-to-vehicle distance, and the driver can easily see, at a glance, which distance is currently set.
The above-referenced Japanese Patent Application Laid-open No. H11-42957 further discloses a lever-like switch as a cruise control switch as illustrated in (a) of FIG. 17. The driver depresses the main switch 400 provided on the side portion of the lever so as to activate the cruise control system. After having accelerated or decelerated the vehicle speed up to the desired speed, the driver can set that moment speed as the set vehicle speed by pushing down and then releasing the lever (namely, by turning the ST/COAST switch to ON). If the driver pulls the lever (namely, turns the CANCEL switch to ON), the cruise control system will be canceled. Besides, after the driver has canceled the cruise control system through some operation upon the brake pedal, if the driver pushes up and then releases the lever (namely turns the RES/ACC switch to ON), the cruise control system will resume the vehicle-to-vehicle distance control.
Since it is easy to add a CC system onto an ACC system, some systems that use alternately an ACC system and a CC system are proposed in recent years. (b) of FIG. 17 shows an example of switches which are currently implemented to realize such systems. The switch shown in (b) of FIG. 17 is basically equivalent to the switch shown in (a) of FIG. 17 but it is additionally provided with a new contact point MODE 410. If the lever is pushed forward for a predetermined time period (namely, the MODE switch is turned to ON) in the state when the vehicle-to-vehicle distance control is not performed by the ACC system, the travel mode will be changed from the vehicle-to-vehicle distance control to the constant vehicle speed control.
Generally, on relatively traffic-congestive roads, the vehicle-to-vehicle distance control is convenient for the driver in terms of decreasing burden of the driver because the adjustments for the vehicle-to-vehicle distance and/or the vehicle speed are performed depending on the environmental road condition when the vehicle travels according to the vehicle-to-vehicle distance control mode. On the contrary, on less traffic-congestive roads, if the vehicle-to-vehicle distance control mode is selected, the driver may encounter an undesired deceleration. Accordingly, in such situation, the driver can travel more comfortably by selecting the constant vehicle speed mode.
However, there is a problem of requiring an extra cost for providing a new contact point onto the conventional cruise control switch as shown in (b) of FIG. 17. Thus, there is a need for such system that can switch the mode between the vehicle-to-vehicle distance control and the vehicle speed control without any extra cost.
Furthermore, when a subject vehicle detects a preceding vehicle traveling at a slower speed than that of the subject vehicle while the subject vehicle is traveling at a set vehicle speed, the subject vehicle may approach to the preceding vehicle keeping that moment speed in case of the travel in the constant vehicle speed control mode, but in case of travel in the vehicle-to-vehicle distance control mode, an automatic deceleration may be performed so as to maintain the set vehicle-to-vehicle distance. Thus, there is no problem even if the subject vehicle actually travels in the vehicle-to-vehicle distance control mode although the driver assumes the constant vehicle speed control mode. However, if the subject vehicle actually travels in the constant vehicle speed control mode although the driver assumes the vehicle-to-vehicle distance control mode, any deceleration action may not be taken as expected by the driver in such occasion that a slower preceding vehicle appears ahead as aforementioned. This may impose a kind of commotion to the driver.
Accordingly, it is desirable that a mode switching from the vehicle-to-vehicle distance control to the constant vehicle speed control should be performed only if the driver explicitly intends such mode switching, so that an unintended mode switching that may be easily caused by wrong or careless operations could be prevented. On the contrary, it is desirable that a simple mode switching from the constant vehicle speed control to the vehicle-to-vehicle distance control should be allowed so as to improve the driver's convenience.